The deposits had been considered possible evidence that extensive lakes or oceans once existed for long periods on the Martian surface before finally evaporating.

The new study concludes that while the region where the deposits were found, known as Meridiani Planum, may at times have contained rivers and ponds in low-lying areas, it was no sea bed (see Mars map).

"Meridiani is one of the few regions of the planet where groundwater would be expected to reach the surface and evaporate to leave behind deposits of salts and evaporite minerals [minerals formed by the evaporation of water]," said lead author Jeffrey Andrews-Hanna, of the Massachusetts Institute of Technology.

His team's study appears in tomorrow's edition of the journal Nature.

Underground River

The researchers used data on Martian topography and geology to model the likely distribution and movement of subsurface water over hundreds of millions of years.

The model shows that water may have traveled distances of up to 621 miles (1,000 kilometers) deep underground before finally spilling out at Meridiani.

The results coincide with other findings, including satellite photographs showing networks of fractured rock through which underground water once flowed